Modern integrated circuits may commonly utilize a combination of both active circuit elements and passive circuit elements to realize circuit designs. For example, FIG. 1 depicts a conventional RF low noise amplifier 100, which may include, for example, passive components such as capacitors 105, 115 and 125 and inductors 107, 117 and 127, and active components such as transistors 112 and 122. The RF low noise amplifier 100 may be formed on a die 102 which can be fabricated using any semiconductor fabrication technique, such as, for example, a Complimentary Metallic Oxide Semiconductor (CMOS) process. Connections with the die 102 may be formed using a wire bond pad for providing signals into and out of the circuit 100. For example pad 120 can provide the RF input signal, pad 130 can provide a supply voltage VDD, pads 140 and 150 can provide ground paths for transistors 112 and 122 respectively, and pad 160 can provide the amplified RF output signal.
Most of the surface area on the die 102 is typically consumed by the passive components. The large consumption of area by passive components is due in part to the large number of passive components included in the circuit, and also due to the fact that passive components can utilize much more surface area than active components. For example, one of the largest components on die 102 is the inductor 107 which may be used for impedance matching. The dimensions of inductor 107 may be approximately 300 μm×300 μm. The dimensions of transistor 112 may only be approximately 0.25 μm×0.25 μm. The inductor 107 therefore may consume well over a million times more area than the transistor 112. Given the high cost of manufacturing using the semiconductor process, having passive components consuming so much die space may be uneconomical. Moreover, because the capability of a circuit typically increases with transistor count, having the passive components taking up the die space which could be more effectively used by a large number of transistors can reduce the desirability of the integrated circuit 100.
One conventional approach to more efficiently utilize semiconductor die space is to fabricate the circuit with multiple dies using three-dimensional flip chip techniques. As shown in FIG. 2A, a low noise RF amplifier 200 may be realized using an active die 204, which may include transistors 112, and 122 formed thereon, and a passive die 202, which may include the inductors 107, 117 and 127, and capacitors 105, 115 and 125. The signal connection between the active die 204 and the passive die 202 may be mechanically formed through contact pads, such as pad 210a and pad 210b. During assembly, the active die 204 shown in FIG. 2A is rotated 290 (i.e., “flipped”) by 180 degrees, and placed on the passive die 202 as shown in FIG. 2B. The joined passive and active dies may be bonded to packaging 206, and electrical connections between the packaging 206 and the passive die 202 may be mechanically realized using wire bonding (e.g., 220 for the RF signal input and 230 for the supply voltage VDD).
As shown in FIG. 2C, the electrical connections for the flip chip integration made between active die 204 and passive die 202 are conventionally performed mechanically using bonded micro bumps 210 at the appropriate contacts pads (e.g., 210a and 210b) on each die. Given the nature of these mechanical contacts, they can introduce additional impedance in the form of additional resistance and capacitance (also referred to herein as parasitic impedance). For many of the components in the RF amplifier 200, this parasitic impedance is not significant. For example, inductors 117 and 127 may be used as filter chokes in the circuit carrying current associated with VDD. Because the low level RF input signal does directly not pass through these components, the RF amplifier is less sensitive to variations in their impedance values. Accordingly, any parasitic impedance introduced by the bonded micro bump contacts associated with inductors 117 and 127 will not typically affect the performance of the RF low noise amplifier 200. However, inductor 107 performs impedance matching for the RF amplifier 200, and because it may be in the direct path of the low level RF input signal, factors affecting the impedance of inductor 107 may adversely affect the performance of the RF amplifier. Such factors will include the parasitic impedance introduced by the bonded micro bump connections associated with the impedance matching inductor 107.
Accordingly, there is a need for coupling techniques used in three dimensional flip chip assemblies which do not introduce parasitic impedances that may affect the performance of integrated circuits.